In life, achieving certain things is good and important; but in most cases, the way they were done or accomplished is just as important, if not more: MLB is no different.
One of the main objectives of the game is to reach base.
However, reaching via a softly-hit infield single is not the same as doing it by hitting a triple or home run, is it?
One is way more valuable than the other because of one crucial aspect: sustainability.
Scouts, talent evaluators, and coaches are trying to determine which players are most likely to produce in the future, and a player who hits the ball hard consistently will be regarded highly, while a slap hitter won’t.
With these things in mind, the MLB sabermetric community has come up with stats to determine the in-game usefulness and sustainability of the way players reach base and produce offensively.
That’s how the concepts of wOBA and xwOBA were born.
What Is wOBA?
wOBA means weighted On-Base Average.
It’s a significant improvement over OBP (on-base percentage) because it gives us a better idea of a player’s offensive contributions using actual outcomes instead of just knowing whether he reached base or not.
A player with two infield singles and two walks in 10 plate appearances will have the same OBP as a guy who hit three home runs and a triple in the same number of plate appearances.
But the production isn’t comparable, is it?
wOBA considers how a player reached base, not only if he did it.
wOBA is a version of on-base percentage that accounts for how a player reached base — instead of simply considering whether a player reached base. The value for each method of reaching base is determined by how much that event is worth in relation to projected runs scored
— Ish (@BPinstripes) November 10, 2018
It gives more weight to things like home runs, triples, and doubles over singles, for example.
This happens because home runs have a higher impact in run expectancy, and should be treated as such.
A single is not the same as a triple in the eyes of wOBA, even if it is to OBP.
What Is xwOBA?
Now that the definition of wOBA is out there, let’s dive into xwOBA, or expected wOBA.
Statcast has, since 2015, provided useful data about exit velocity, launch angle, and similar concepts.
xwOBA uses Statcast data to determine a player’s expected wOBA based on quality and quantity of contact.
What does this mean?
Per MLB’s explanation, “each batted ball is assigned a single, double, triple and home run probability based on the results of comparable batted balls since Statcast was implemented Major League wide in 2015.”
A 110-mph line drive hit at 25 degrees will have more weight in the eyes of xwOBA than a five-degree chopper to shortstop, even if the latter finds its way to the outfield.
xwOBA helps us understand what should be the player’s wOBA based on the quality and quantity of contact.
What’s The Difference Between The Two?
The main difference between the two is that wOBA helps us understand a player’s offensive contributions judging it by outcomes, whereas xwOBA considers deserved outcomes.
How do we know if a batted ball “deserves” to be regarded highly?
By using Statcast metrics such as exit velocity and launch angle.
xwOBA evaluates quality of contact, wOBA looks at actual outcomes, and OBP only cares if the player reached or not.
xwOBA has a lot of descriptive value, and some predictive value, as well.
200 hitters have at least 75 PA. The top 15 of the xwOBA leaderboard is filled with the studs you would expect… and also, good lord, Mitch Haniger, what are you doing? pic.twitter.com/XjeN0qgMzR
— Mike Petriello (@mike_petriello) May 2, 2018
The best players in baseball have a high xwOBA.